This invention relates to an optical scanning device comprising a radiation detector array for performing both focus error detection and radial tracking error detection, to such a radiation detector array and to a method for adjusting such an optical scanning device.
One optical recording and playback system under development is the DVR (Digital Video Recording) system. The system uses a relatively high numerical aperture (NA) objective lens, of 0.85, compared to the conventional Compact Disk (CD) NA of 0.45 and the conventional Digital Versatile Disk (DVD) NA of 0.60, a xe2x80x9cbluexe2x80x9d wavelength of approximately 400 to 430 nm, and recording on groove tracks alone or on both land and groove data tracks.
Various different methods are known for focus error detection and radial tracking error detection. The focus error detection methods include knife edge pupil obscuration, in which the beam is split into two by e.g. a prism and the position of the spots on two spot detectors indicate correct focusing; astigmatic focusing, in which an astigmatic spot on the detector is created by means of a cylindrical lens or a plane parallel plate, and variations in the shape of the spot from circular are detected by a diamond-shaped quadrant spot detector; and spot size detection, in which the beam is separated into two by e.g. a microprism and detecting the resulting spot sizes before and after refocusing respectively.
The radial tracking error detection methods include push-pull radial tracking, in which a difference in signal between two pupil halves are measured on separate detectors; three spot (or three beam) central aperture radial tracking, in which the radiation beam is split into three by a diffraction grating, and the outer (satellite) spots are set a quarter track pitch off the main spot and the difference of their signals used to generate the tracking error signal; three spots push-pull radial tracking, in which the radiation beam is split into three by a diffraction grating and a difference between the push-pull signals of the main spot and the satellite spots is used as the tracking error signal; and Differential Phase or Time Detection (DPD or DTD) radial tracking, in which the contribution of the radial offset of the phase of the (xc2x11,xc2x11) orders is exploited in a square-shaped quadrant spot detector. The three spot push-pull radial tracking system has the advantage over one spot push-pull systems is that systematic errors, including symmetric errors and asymmetric errors, may be compensated for automatically. However, the system requires additional detector elements and connections, increasing the complexity of the detector array. A DPD radial tracking method is used in certain systems, such as DVD+RW and DVDxe2x88x92RW scanning devices. DVD-ROM and DVD-video also use DPD or DTD radial tracking.
In the case of a DVR system using push-pull tracking, a relatively large modulation of the DVR optical signal reduces the effectiveness of the conventional astigmatic focus error detection method, because of its sensitivity to radial-to-focus crosstalk.
EP-A-0523319 describes an optical pickup unit including a detector array arranged such that a push-pull method is used for detecting a tracking error, and the focal point is detected by measuring a beam size. Only a single spot detector is used, even in a three beam method which is suggested as a possible alternative method of detecting a tracking error.
A known arrangement of optical pick-up unit, produced by Sony and known as the xe2x80x9cLaser Couplerxe2x80x9d, has a detector array which combines one spot push-pull radial tracking with spot size detection focusing.
Other examples of the use of spot size detection focusing and one beam push-pull tracking are described in U.S. Pat. Nos. 5,278,401, 5,347,504, JP-A-5-266530, JP-A-4-229435, JP-A-5-274684 and JP-A-5-62221.
In accordance with one aspect of the present invention there is provided a radiation detector array comprising a plurality of spot detectors for detecting radiation beams scanning an optical record carrier in the form of a disk having a tangential direction in which tracks are arranged and a radial direction orthogonal thereto, each said spot detector being arranged to detect a characteristic of a spot formed by a said beam and each said spot detector comprising a plurality of detector elements for detecting different parts of a said spot, wherein said detector array is for radial tracking error detection and focus error detection, said array comprising:
a first plurality of spot detectors for conducting multiple-spot radial tracking error detection, said first plurality of spot detectors including a first spot detector arranged substantially centrally thereof; and
a second spot detector for conducting spot-size focus error detection in combination with said first spot detector.
This aspect of the invention makes it possible to combine a multiple-spot radial tracking error method with spot-size focus error detection efficiently, using a relatively small number of detector elements and connections.
Preferably, said second spot detector is arranged in a position offset to one side of said first line. More preferably, said second spot detector is arranged in a second line with said first spot detector, which second line is arranged at an angle of at least 15xc2x0 with respect to said first line. By such an arrangement, outer spots, produced by a diffractive element for the multi-spot tracking error detection, are inhibited from interfering with the focus error signal detected by the first and second spot detectors. In particular, higher order spots, which extend in the direction of the first line, are inhibited from interfering with the desired signal detection.
Preferably, the first spot detector comprises a plurality of detector elements separated by separation means, said separation means comprising a first separation means defining a signal separation for use in said radial tracking error detection and a second separation means defining a signal separation for use in said focus error detection, wherein said first and second separation means are arranged substantially parallel. This arrangement tends to reduce the number of detector elements required to conduct both radial tracking and focus error detection.
In this parallel arrangement, the second spot detector is preferably arranged in a second line with said first spot detector, which second line is arranged at an angle of less than 75xc2x0 with respect to said first line. This allows the adjustment of an optical scanning device during manufacture to correctly centre the spot on both the first and second spot detectors for spot size detection, by azimuthal rotation of the optical component producing the spot separation.
In an alternative arrangement, the first spot detector comprises a plurality of detector elements separated by separation means, said separation means comprising a first separation means defining a signal separation for use in said radial tracking error detection and a second separation means defining a signal separation for use in said focus error detection, wherein said first and second separation means are arranged substantially orthogonally. Whilst not as efficient in terms of the number of detector elements included, this alternative arrangement also provides an otherwise efficient arrangement.
A radiation detector array according to any preceding claim, wherein said second spot detector comprises less detector elements than said first spot detector.
The second spot detector is preferably arranged not to participate in said radial tracking error detection. This reduces the number of detector elements and connections required in the detector array.
Alternatively, fifth and sixth spot detectors may be arranged generally linearly along a further line substantially parallel to a radial equivalent direction and having said second spot detector arranged substantially centrally thereof. This can be used to improve the efficacy of radial tracking.
A radiation detector array according to any preceding claim, wherein said first and second spots are adapted to jointly detect a data signal containing data recorded on the optical record carrier.
In accordance with a second aspect of the invention, there is provided an optical scanning device comprising a radiation detector array as described,
the device comprising a diffractive component for generating multiple beams arranged in a line of diffraction, and a beam separating component for separating at least one of said multiple beams into two further beams in a direction having at least a component of orthogonality with respect to said line of diffraction. The device provides the appropriate beam configuration for the detector array of the first aspect.
In accordance with a further aspect of the invention, there is provided a method comprising adjusting said beam separating component in position with respect to said detector array by azimuthal rotation about an axis of said at least one of said multiple beams. This allows the centring of the spots with respect to the spot size detection arrangement.